There are still some other parameters which need to be determined. In the numerical analysis, we adopt the masses of the charm quark, the neutron and the \(\Lambda _c\)  baryon as \(m_c=1.27\,\mbox{GeV}\) , \(M=m_n=0.939\,\mbox{GeV}\)  and \(M_{\Lambda _c}=2.286\;\mbox{GeV}\) . The coupling constant \(f_{\Lambda _c}\)  defined in Eq. (REF ) is another important parameter to be determined. In the following analysis we use the estimate obtained in Ref. [1]}: \(f_{\Lambda _c}=(9.1\pm 0.2)\times 10^{-3}\;\mbox{GeV}^2\) . In addition, two artificial parameters, the continuous threshold \(s_0\)  and the Borel mass \(M_B^2\) , need to be determined. The threshold \(s_0\)  is the parameter to be used to represent the continuous contribution by integrating the spectral density in the area above it. In the viewpoint of the physical content, the threshold is related to the first excited state of the inserted ground hadron, the \(\Lambda _c\)  baryon. Additionally, the result should not depend on the threshold two much. In the work, we choose the value in the region \(7.5\,\mbox{GeV}^2\le s_0\le 8.5\,\mbox{GeV}^2\) . Another parameter, the Borel mass, is determined with the requirements that both higher twist contributions and higher resonance contributions are suppressed, and the sum rules do not depend on the Borel mass too much in the working region simultaneously. Making use of the light-cone distribution amplitudes presented in Ref.[2]}, [3]}, [4]}, we show the dependence of the form factors on the Borel parameter at the squared momentum transfer \(q^2=0\,\mbox{GeV}^2\)  in Fig. REF . The results indicate that in this window the sum rules of the form factor \(f_1(0)\)  meets the above requirements and are flat on the Borel parameter. However, the form factor \(f_2(0)\)  depends much on the Borel mass in the same region. The working window of Borel mass is thus chosen as \(7.5\,\mbox{GeV}^2\le M_B^2\le 8.5\,\mbox{GeV}^2\) . To make a more reliable estimate of the decay mode, we plot the \(q^2\)  dependence of the form factors \(f_1(q^2)\)  with the Borel mass being chosen as \(M_B^2=8.0\,\mbox{GeV}^2\)  and \(f_2(q^2)\)  with the three different points of Borel mass \(M_B^2=7.5\,,8\,,8.5\,\mbox{GeV}^2\)  in Fig. REF .
<FIGURE><FIGURE>